In­com­plete dom­i­nance

AVERY good day to you all! How are you? What a won­der­ful time we are go­ing to have togther! Guess what? If you waste it, you will never get an­other chance to make up for the wasted hours, so use them wisely.

Ge­net­ics is fun if you un­der­stand what is be­ing done and if you prac­tise as you go along. It can be easy to learn but also easy to for­get, so stay con­nected!

This week, we will look at the phe­nom­e­non known as in­com­plete dom­i­nance. So far we have looked at two types of dom­i­nance, com­plete and co-dom­i­nance. Do you re­mem­ber what these are? Of course you do! In com­plete dom­i­nance, one al­lele is dom­i­nant over the other and cross­ing the het­erozy­gotes re­sults in a phe­no­typic ra­tio of 3:1. In co-dom­i­nance, two dom­i­nant al­le­les come to­gether and both are ex­pressed in the phe­no­type, for ex­am­ple, the blood group AB. In in­com­plete dom­i­nance, this does not hap­pen. When­ever the dom­i­nant and the re­ces­sive al­le­les come to­gether, a third phe­no­type is pro­duced. This will result in a phe­no­typic ra­tio of 1:2:1. Let us see how this hap­pens.

This phe­nom­e­non is shown in the flower colour in the plant, Im­pa­tiens. The flower colour is ex­pressed in 3 phe­no­types, red, pink and white. There is an al­lele that rep­re­sents red colour and one that rep­re­sents white. We can use the sym­bol R to rep­re­sent the red al­lele and we can use the sym­bol r to rep­re­sent the white al­lele. Let us look at what hap­pens when a red flower is crossed with a white. (See Fig­ure 1) Geno­typic ra­tio: All het­erozy­gotes - Rr

Phe­no­typic ra­tio: Here is where the sur­prise comes, these flow­ers in­stead, of be­ing red, are pink! So our phe­no­typic ra­tio would be: All pink. Let us see what hap­pens when we cross the F1, the het­erozy­gote. (See Fig­ure 2)

What is our geno­typic ra­tio? Is your an­swer 1:2:1? If it is, then you are cor­rect! What is the phe­no­typic ra­tio? If this were a nor­mal case, then it would be 3:1 or three red to one white, but this is not the nor­mal case. Re­mem­ber the phe­no­type of the par­ents? Yes, you do. It was pink! So, know­ing this, can you work out the phe­no­typic ra­tio? Of course you can. It is like the geno­typic ra­tio, 1:2:1 or 1 red (RR): 2 pink (Rr):1white (rr). If you are given these re­sults in an exam and are asked to state the con­di­tion, then you would be able to name it as in­com­plete dom­i­nance. That com­pletes our study on dom­i­nance; not too bad, was it? We switch now to an­other type of di­a­gram used in ge­net­ics. This is the pedi­gree chart. What is a pedi­gree chart? A pedi­gree chart is used to show how a trait is passed on or trans­mit­ted in a given fam­ily. It can be used to show how the trait is passed on and also to pre­dict the prob­a­bil­ity of the geno­type of future off­spring. In con­struct­ing a pedi­gree chart, cer­tain sym­bols and con­ven­tions are used: Sym­bols are used to rep­re­sent peo­ple and lines are used to rep­re­sent re­la­tion­ships. Males are rep­re­sented by squares and fe­males are rep­re­sented by cir­cles. Hor­i­zon­tal lines con­nect­ing a male and fe­male rep­re­sent mat­ing. Ver­ti­cal lines lead­ing down­wards from the hor­i­zon­tal line rep­re­sent the off­spring.

The sym­bols at the top of the chart rep­re­sent the old­est in­di­vid­u­als of the gen­er­a­tions. Sub­se­quent gen­er­a­tions are writ­ten un­der­neath the first parental gen­er­a­tion.

When analysing the pat­tern of in­her­i­tance of a par­tic­u­lar trait, it is cus­tom­ary to shade in the sym­bol of all in­di­vid­u­als that pos­sess this trait. Below is a very sim­ple ex­am­ple of a pedi­gree chart. No­tice the fol­low­ing: There are two off­spring pro­duced in the F1, one male (square) and one fe­male (cir­cle). In the F1, the male has in­her­ited the trait. The male with the trait has mated with a nor­mal fe­male There are four off­spring in the F2, three males and one fe­male.

Three of the off­spring, two males and one fe­male do not have the trait while one male has the trait.

Pedi­gree charts are some­times given in the ex­ams, so learn the con­ven­tions. Can you con­struct one for your fam­ily? See you next week!